This invention relates to PLZT shutters such as utilized in light modulators and displays, and more specifically relates to space-charge problems associated with such devices.
PLZT is an abbreviation for a lanthanum modified lead zirconate-lead titanate ferroelectric ceramic material which is optically transparent This material and its application as a shutter or light modulator is generally well known; for example, see the article entitled "PLZT Electrooptic Ceramics and Devices" by Dr. Gene H. Haertling, ACS Symposium Series No. 164, Industrial Applications of Rare Earth Elements, 1981. By controlling the magnitude of an electric field induced in a plate of PLZT disposed between two crossed polarizers, a shutter can be switched between ON and OFF states.
External light entering a PLZT shutter is polarized by the first polarizer and, with no electric field applied to the PLZT plate, strikes the second crossed polarizer substantially out of phase so that virtually no light is transmitted through the second polarizer. This defines the OFF state. Applying an electric field to the PLZT plate causes a phase retardation of the polarized light passing through it. By providing a phase retardation of approximately 90.degree., the polarized light is substantially in phase with the second polarizer and easily passes through it. This corresponds to the ON state.
Operation of a PLZT shutter, especially for long periods of time in the ON state, may result in an undesired condition wherein the removal of the externally induced electric field o the PLZT plate does not cause the shutter to completely turn OFF. Varying amounts of light leakage through the shutter usually in irregular patterns or locations result thereby lowering the ON-OFF ratio and creating problems in applications where light transmission in the OFF state must be very low. This problem is hereinafter referred to as a space-charge problem.